1. Field of the Invention
The present invention relates to an image display device and, more specifically, to an image display device that is capable of increasing utilization efficiency of light emitted from a laser diode light source and reducing the effect of speckle noise.
2. Description of the Related Art
As an image display device that projects image display light from an optical unit such as a lens so as to present an image to a user, a so-called “projector” has been conventionally known widely. In recent years, new needs have started being established for this projector such as needs for easy portability by a user, usage while being connected to a portable terminal owned by the user, and the like. Therefore, there is an increasing demand for the miniaturization of the projector.
As an image display device that presents an image based on image display light, so-called “head up displays” have been developed conventionally. Head up displays have an optical element called combiner. This combiner allows external light to pass through and reflects image display light projected from an optical unit provided in a head up display. This allows the user to visually recognize an image related to the image display light while overlapping the image on a landscape via the combiner.
Head up displays have received attention as in-vehicle image display devices in recent years since the head up displays allow a driver of a vehicle to also recognize, almost without changing the line of sight or a focus for visually recognizing a view outside the vehicle, information of an image projected from an optical unit. There is also an increasing demand for the miniaturization of these head up displays due to the need for installation in a limited space such as a vehicle.
Thus, instead of discharge lamps such as ultrahigh pressure mercury lamps, xenon lamps, or halogen lamps used as light sources in conventional projectors, semiconductor light sources that allow for further miniaturization of the devices have been developed. In comparison with discharge lamps, semiconductor light sources have advantages such as low power consumption, instantaneous lighting capability, long life, high chromatic purity, or non-use of mercury, and the advantages are also factors that encourage the development. Among semiconductor light sources, light emitting diodes (LEDs) are spreading rapidly. For example, Patent document No. 1 describes an image display device that uses an LED as a light source.
[Patent document No. 1] Japanese Patent Application Publication No. 2008-026853
It is necessary to increase power to be supplied or increase the area of a light-emitting surface in order to obtain, using an LED as a light source, the same degree of brightness as the brightness obtained when a discharge lamp is used as a light source. Therefore, there has been a problem where it is difficult to obtain sufficient brightness when an LED is used as a light source in a small image display device.
Thus, a possible option is to use a laser diode (LD) as a light source. Different from an LED, a laser diode is not a surface light-emitting source and is almost close to a point light-emitting source. Therefore, compared to an LED, a laser diode allows for the miniaturization of a device while increasing the amount of beam rays per unit area (luminous flux).
However, in general, a beam emitted from a laser diode has deviation (astigmatic difference) in the spread thereof. FIG. 14 shows distribution of the intensity of light emitted from a laser diode. As shown in FIG. 14, the distribution of the light emitted from the laser diode spreads wider in one direction (a Y direction in FIG. 14) than in another direction that is perpendicular to the direction. In other words, there is deviation (astigmatic difference) in the spreading of the beam. FIG. 15 schematically shows light emitted from a laser diode. As shown in FIG. 15, the light emitted from the laser diode does not spread in a circular shape but spreads in a substantially elliptic shape where the light spreads wider in the Y direction compared to the X direction.
When light is emitted spreading in a substantially elliptic shape as described above, there is a problem where the utilization efficiency of the light as image display light becomes lower than light that spreads in a substantially circular shape. A decrease in the utilization efficiency of the light may cause an increase in the size of a device and a decrease in the brightness of a projected image.
Further, the use of a laser diode as a light source causes a problem where speckle noise is generated by interference light due to high coherence of laser light. The quality level of a projected image may be lowered due to the generation of speckle noise.